An amplifying optical fiber having an axially symmetric waveguide structure in which a core and/or a cladding has been doped with rare earth elements are used as optically-active media of fiber amplifiers and fiber lasers. In particular, Yb-doped optical fibers which contain ytterbium (Yb) as the rare earth element can obtain high-power output light with excellent beam quality. The oscillation wavelength of this output light is the wavelength around 1 μm which is substantially the same as Nd—YAG laser which is a conventional type of existing high output laser. Because of this, practical applications of Yb-doped fiber lasers as a high output light source laser medium are expected to be material processing such as welding, marking, cutting and the like.
FIG. 12 shows an example of a cross section in the radial direction of a conventional Yb-doped optical fiber and of the refractive index profile thereof.
An Yb-doped optical fiber 110 shown here is a single cladding fiber in which cladding 112 is provided on an outer circumference of a core 111, and a protective coating layer 113 is provided on an outer circumference of the cladding 112. In this Yb-doped optical fiber 110, the refractive index of the core 111 is higher than the refractive index of the cladding 112, in order for guided lights to be confined. Normally, a refractive index-raising dopant such as germanium (Ge), aluminum (Al), or phosphorus (P) is doped into the core 111, in order to raise the refractive index of the core 111. Furthermore, the core 111 is also doped with a dopant Yb which has an optical amplifying function. Yb is normally doped into the core 111 so as to have a substantially uniform density distribution. However, it is acceptable if there is some density distribution and it is also possible for a portion of the cladding 112 to be doped.
A high-power signal light can be obtained by making pumping light incident on this Yb-doped optical fiber and then making signal light incident thereon, or by making pumping light incident on this Yb-doped optical fiber and then consisting cavities using a fiber Bragg grating.
Normally, when an Yb-doped optical fiber is used as an optical amplifying medium in a fiber laser or fiber amplifier, the Yb-doped optical fiber is mostly used in an effectively single-mode condition in order to utilize the advantages of a fiber-type optical amplifying medium, namely, the ability to realize restricted propagating condition together with excellent cooling efficiency to be utilized.
The conditions of an optical waveguide which enable effectively single-mode propagation to be achieved are determined by conditions of the refractive index of the core and the core diameter (namely, refractive index profile in the radial direction of the core) and the winding diameter of the fiber and the like. At this time, it is necessary either the core have a low refractive index or the core diameter is small.
In contrast, in view of the properties of an optical amplifying medium, it is desirable for it to be possible to output even higher-power light. Namely, making it possible for high-power light to be propagated through an optical fiber is a requirement for a better amplifying optical fiber. However, in the case when a same light power is incident into an optical fiber having a small core diameter, then compared with the case when such light is incident into an optical fiber having a large core diameter, because the light-propagating cross-sectional area (i.e., the mode field diameter) in the former case is smaller than that in the latter case, the power density of the light being propagated through the core is increased. As a result, damage in the core glass due to the light and non-linear optical phenomena is easily induced. Alternatively, the amplified power during light propagation becomes restricted. Accordingly, for reasons such as these, larger core diameters are desirable. Consequently, in order to enlarge the core diameter and enable single-mode propagation to be realized, it is necessary to lower the core refractive index.
One of the factors which cause the properties of a fiber amplifier or fiber laser to deteriorate is loss increase (i.e., photodarkening) of an optical fiber which is caused by the pumping light or signal light propagated through the optical fiber (see Non-patent documents 1 and 2). Because of this loss increase, the optical amplifying efficiency of the rare earth-doped optical fiber serving as the light amplifying medium is gradually decreased. As a result, the output of the fiber amplifier or fiber laser is decreased over time and the product lifetime thereof is accordingly shortened.
Thus, various methods for suppressing photodarkening have been disclosed up to this time.
For example, in Non-patent document 1, a method is disclosed in which photodarkening is suppressed by employing a special manufacturing method known as DND (Direct Nanoparticle Deposition).
Moreover, in Non-patent document 2, a method is disclosed in which photodarkening is suppressed by doping a high concentration of aluminum into an optical fiber during the manufacturing thereof.
Moreover, in Patent document 1, a method is disclosed in which photodarkening is suppressed by doping hydrogen into an optical fiber.